Optical glass fibers used as a medium for optical transmission (hereinafter referred to as "optical fibers", for simplicity) generally have a diameter of 200 .mu.m or less and are made of brittle materials so that cracks tend to occur on the surface during the manufacturing process of the fibers, production step of the fibers into a cable or storage of the cable. Any crack on the fiber surface becomes a source of stress concentration and the fibers easily break upon application of an external stress. Thus, it is extremely difficult to directly use optical fibers as a medium for optical transmission.
Many attempts have been made to form a plastic coating on the surface of optical fibers, thereby producing optical fibers maintaining the initial strength and having durability in use for a long period of time as described, for example, in T. T. Wang et al., J. Appl. Poly. Sci., 23, 887-892 (1979).
Such a plastic coating comprises a first coating to maintain the initial strength, a second coating to withstand subsequent handling (e.g., assembling into a cable) which is formed by extrusion molding a thermoplastic resin such as polyamide or polyethylene and a buffer layer provided between the first and second coatings to minimize the transmission loss that may occur during the application of the second coating. The first coating and buffer layer are generally formed in the step subsequent to the drawing step of the optical fibers.
However, the thermosetting resins such as urethane and epoxy resins conventionally used to make the first coating have slow curing speed and require a long period of time to cure and dry so that the drawing rate of the optical fibers is limited, causing one of problems on mass production of the optical fibers. Another disadvantage is that these resins cannot be applied in a thick coating to provide the optical fibers having a sufficient strength. Furthermore, the buffer layer is conventionally made of RTV silicone rubber to minimize transmission loss, but this silicone rubber increases its viscosity by merely allowing to stand it at room temperature and cannot withstand drawing for a long period of time. In addition, the silicone rubber has relatively high tackiness so that dust and dirt easily adhere on its surface thereby adversely affecting at the application of the second coating.